The invention relates to a method for representing variable information on a display device, in particular in a motor vehicle, having a first, dot matrix display having a liquid crystal cell, and having a second display having a liquid crystal cell, said second display being arranged optically in series with the first display, and the liquid crystal cell of the first display being driven using a multiplex method.
Methods of this type which serve to represent information on liquid crystal displays are known. In this case, two identical liquid crystal displays arranged optically in series are used in order to obtain a high image resolution and a high information density of the display by virtue of the superposition of representations of the two displays.
With the multiplex rate which is necessary for the driving in a multiplex method and rises with the number of image rows and columns present, not only does the necessary drive voltage change but also the switching time of the respective display, said switching time resulting from the sum of switch-on time and switch-off time. In this case, the switch-on time encompasses the period of time which elapses from the first drive pulse of a driven display segment until maximum contrast is reached between the driven character and its surroundings; the switch-off time, on the other hand, encompasses the period of time which passes from the cessation of the drive pulse until minimum contrast is reached between the character and its surroundings. The switching time of the display may be dependent on the liquid crystal substance used, the drive voltage and the driving method. Furthermore, there is a great dependence on the ambient temperature such that with falling ambient temperatures, the switching time of the display becomes very long. Therefore, it is not possible to reliably represent information on a display having a large number of dots at low ambient temperatures. This proves to be unacceptable particularly when safety-relevant information is represented by the display.
In order to remedy this deficient state of affairs, it is known, therefore, to provide a light box, serving to illuminate a liquid crystal cell in a liquid crystal display, with a heating wire. By means of the heating wire, the liquid crystal cell is heated and kept at a temperature level which enables the liquid crystal display to be operated with the requisite short switching times even at low ambient temperatures. On account of the relatively large distance between the heating wire and the liquid crystal cell, a high power supply and great heating of the heating wire are necessary in order to be able to generate the necessary heating of the liquid crystal cell. Furthermore, the heating wire heats the entire display unit, in an undesirable manner, on account of its arrangement in the light box and its required high radiation of heat.